Black Cat!
Well, the beginning chapter/one or two volumes are not that good
but that's the amazing part
because you can see the mangaka's development
and by the end of Black Cat, the drawings are INCREDIBLE

ayashi no ceres(well, any of yuu watase's later works)
pretty much anything by clamp(they have great, unique costume ideas. i like how in tsubasa it looks edgey yet elegant at the same time)
love com(i like a lot of their clothes :])
vampire knight
godchild
crimson hero(i like how they look realistic, yet they still have that "anime look" to it)
full moon wo sagashite(so much detail but it looks really good in her mangas)

These are all I can think of at the moment:
Vagabond
Air Gear
Tenjou Tenge (Not in the beginning but later on)
Ah! My Goddess (artwork was also bad in the beginning but became very good later on)
One Piece (yes, I believe that One Piece has very good artwork. It isn't omg it's so pretty artwork, but Oda's style has its own charm.)
Any manga drawn by Obata Takeshi (ex. Hikaru no go, Death Note, Blue Dragon)
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